1. Technical Field
The present invention relates generally to electrically fired explosive devices, and more particularly to control elements for protecting small electrically fired, fixed case ammunition from the hazards of unintended firing due to stray radio frequency energy.
2. Background Art
Electroexplosive devices such as electric blasting caps, squibs and detonators are used in many contexts, such as blasting operations, ammunition and the like. Electroexplosive devices include at least one electrical ignition device disposed in ignition relationship with one or more heat-sensitive explosive charges, such as first fire mixtures, and are fired by passing a D C current through a pair of leads connected to a filament or bridge of high electrical resistance which is in heat transferring contact with the first fire mixture. A sufficient flow of current heats the bridge wire to indescance thereby igniting the surrounding mixture. The energy generated from ignition of the mixture is then used to ignite a sequence of pyrotechnic and/or explosive charges which in turn can ignite or detonate other charges.
These electroexplosive devices are subject to unintended discharge by stray electromagnetic or electrostatic energy. Therefore, electric firing techniques have included procedures intended to minimize this possibility and to protect individuals in the vicinity of these devices. However, the value of such precautionary measures is diminished because it is difficult to predict the extent of the electromagnetic radiation hazard from one moment to the next and the levels of electrical hazards are steadily increasing.
Prior attempts at solving the problems associated with electroexplosive devices caused by stray radio frequency energy have included decreasing the sensitivity of the bridge by designing the bridge to require very high firing currents for igniting the pyrotechnic chemical disposed adjacent to that bridge. This approach requires the use of heavy and expensive wiring and requires the use of power sources providing high energy levels. In addition to the increased expense associated with this approach, this approach still fails to provide adequate safety.
In the past, most electroexplosive devices that are suitable for use in radiation hazards environments have used a filter and heat sink combination. The filter attenuates the radiation and the heat sink transfers heat generated during attenuation away from the bridge wire and explosive components. One such filter is disclosed in U.S. Pat. No. 4,378,738 of Proctor et al. of a common assignee. However, the '738 devices are too large to be compatible with fixed or semi-fixed ammunition and therefore extensive modifications would have to be made in order to adapt these devices to a small size. Such modifications are unacceptable for many reasons, including a concomitant requirement to alter various procedures associated with the manufacture of such devices and perhaps a requirement to alter existing firing circuits. Furthermore, to be effective, the filter must be coupled closely to the bridge wire leads and shielded from electromagnetic radiation leakage paths. Furthermore, in fixed case or semifixed ammunition unit cost is extremely important. Under many conditions, the heat sink associated with the presently known devices may not be large enough. However, adding external heat sinks may be impractical due to size, cost and use considerations. Furthermore, since the explosive output of the device is usually buried in a booster or explosive, there may be no available area for an additional heat sink.
Due to these problems, it has been proposed to use ferrite beads to attenuate radio frequency energy. However, such approaches require use of capacitors in order to obtain broadband attenuation and even then the low frequency attenuation may be unacceptable. In such a case it was necessary to use a plurality of ferrite beads in series along each of the two electrical leads with capacitors connected between junctions of corresponding beads of the two leads thus forming a low-pass attenuation network. Still further attenuation problems arise because the ferrite used in these devices had low curie temperatures so that attenuation of even moderate radiation caused sufficiently high temperatures to vitiate the attenuation properties of the device. The use of capacitors, itself creates problems because the combined device and capacitor is too bulky to fit a small primer pocket. Even then, it is questionable whether a single capacitor will provide the device with the capability to cover a frequency spectrum of from about one megahertz to about eleven gigahertz as is required to include the known hazards.
Accordingly, there is a need for a device which is effective in protecting small devices against the hazards associated with stray electromagnetic energy in a cost-effective manner.